The present invention relates generally to particle filters, and more particularly to microfabricated thin-film particle filters.
Precise control of filter pore sizes in the 50 to 100 angstrom range, for either organic or inorganic filters, would allow, for example, biologically important molecules to be mechanically separated on the basis of size. In the present state of the art, there is a very limited selection of filters having pores much less than the resolution limit of 0.35 microns of photolithography. The filters known heretofore having pore sizes in this range include polycarbonate membrane filters, sintered filters, zeolites, and one instance of a microfabricated bulk micromachined filter.
Polycarbonate membrane filters (nucleopore filters) may be used where pore sizes between 500 and 3500 angstroms are needed. These filters, however, cannot be used at high temperatures, in strong organic solvents, or where no extracted oligomers can be tolerated. The pores of polycarbonate membrane filters are also randomly located. As such, there is a compromise between having a high enough population of pores per unit area and having too many instances of partially overlapping pores. Partially overlapping pores provide pathways through the filter that allow some particles to get through that are larger in diameter than the rated cut-off size of the filter.
Filters that are available in other materials, such as metals or ceramics, are made by sintering together discrete particles. This technique yields a random structure with a relatively large dead volume and no exact cut-off size above which transport is impossible.
Materials such as zeolites, which have a crystal structure with large channels, can be used as molecular sieves in the limited range of from about 5 to 50 angstroms. Zeolites are not amenable to fabrication as thin membranes.
A microfabricated filter with bulk micromachined structures is described by Kittilsland in Sensors and Actuators, A21-A23 (1990) pp. 904-907. This design uses a special property of silicon wherein silicon becomes resistant to a certain etchant when highly doped with boron. The pore length of this filter is determined by the lateral diffusion of boron from a surface source at a single crystal silicon-thermal oxide interface. As is known, such lateral diffusion is the diffusion in the plane of the source, away from the source. The use of this technique makes it very difficult to precisely control the pore length. This filter also does not consist entirely of thin films. i.e. films grown on a substrate by techniques such as cathode sputtering, vacuum evaporation and chemical vapor deposition (CVD). Rather, the fabrication of this filter requires bulk micromachining, including extensive etching of the substrate. As a result, the filter is difficult to integrate with other microfabricated devices. Also the method of fabricating this filter cannot be applied to materials other than silicon.
Thus, it can be seen that currently there are no filters that can be fabricated from a wide variety of materials and that have pores of well controlled shape and size smaller than about 3500 angstroms and arranged in a precise pattern to exclude the possibility of overlap.
Accordingly, an object of the present invention is to provide a thin film inorganic membrane filter having geometrically defined pores of submicron size, with precisely controlled width and length.
Another object of the present invention is to provide thin film diffusion barriers for controlled time release of reagents or medications.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.